The present invention relates to an induction heater for induction heating heated steel materials that are continuously carried.
In an iron and steel rolling line, generally, heated steel materials are heated to a predetermined temperature in advance, and the heated steel materials are continuously carried to a plurality of rolling mills. The rolling mills sequentially roll the heated steel materials into thin plates. In this rolling process, the heated steel materials radiate heat and are gradually cooled down during carrying. The rolling of the heated steel materials in this state has a quality problem. Therefore, the heated steel materials are heated again before the finish rolling. Thus, the heated steel materials are rolled after the whole heated steel materials have been heated to a high temperature.
For heating the steel materials, a whole heater (a coarse bar heater) formed in a solenoid coil is passed through the heated steel materials to increase the temperatures at all portions of the heated steel materials. Particularly, as the temperatures at both end portions of each heated steel material become lower than the temperature at the center of the heated steel material, a C-shaped inductor is used to locally heat both ends of the steel material. Thus, the temperatures at all portions of the heated steel material are made uniform. The rolling is carried out after this process.
As shown in FIG. 1, a C-shaped inductor 1 for locally heating both ends of the steel material has upper and lower iron core legs 3, 3 formed to sandwich a gap 4 of a C-shaped iron core 2, with heating coils 5, 5 wound around both iron cores. An end portion of a belt-shaped steel material 6 is passed through the gap 4 of the C-shaped inductor 1, and a current is flown through the heating coils 5, 5 from a power source. Thus, a magnetic flux .PHI. is generated in upward and downward the iron core legs 3, 3. This magnetic flux .PHI. is interlinked to the heated steel material 6, thereby to induce an eddy current. As a result, Joule heat is generated to heat both end portions of the heated steel material.
According to an induction heater using this C-shaped inductor 1, the heated steel material 6 is supported by table rollers 9 and carried. These table rollers 9 are earth-connected via roller stands 10 respectively. Therefore, an induced current for contributing to the heating flows to the earth through the table rollers 9 of the rolling facility.
An equivalent circuit in this state will be explained with reference to FIG. 2. A magnetic flux .PHI. interlinks to a loop circuit 7 formed by a resistor R2 of the heated steel material 6 continuously carried and a resistor R1 of an edge portion and to a loop circuit 8 formed by a ground resistor R0 and the resistor R1 of the edge portion. Thus, the alternating magnetic flux .PHI. interlinks within the closed loop to generate electromagnetic induction. Based on this principle, electromotive force of E=-d.PHI./dt is generated. As a result, induced currents I1 and I2 flow through loop circuits 7 and 8.
With the above arrangement, the current I2 that flows to the earth via the table rollers 9 generates a spark at a contact point between the heated steel material 6 and the table rollers 9. When the level of power applied to the heating coils 5, 5 is high, an arc hurt is generated on the heated steel material 6, which results in a poor finished product.
In order to solve this problem, there has been a method of interrupting a current that flows to the earth, by sandwiching an electric insulating material into the roller stands 10 that support the table rollers 9 and installing the table rollers on the floor.
However, the provision of the insulation processing in the roller stands 10 over the range of a few hundred meters of the rolling facility has had a problem that the introduction of this facility requires a large amount of cost. Further, there has been a problem that oxidized scales are dispersed and are adhered onto the surface of the insulation materials during the use of the facility. This causes an insulation failure.
Further, as another measure against the above problem, there has been a method of setting an electromotive force to zero as shown in FIG. 3. According to this method, two C-shaped inductors 1, 1 are installed in parallel within the closed loop between the roller stands 10, 10. Directions of the magnetic fluxes of the two C-shaped inductors 1, 1 are inverted to set the electromotive force to zero.
However, according to this method, there is a limit to the size for installing the facility. It is not possible to employ this method at a place where the two inductors 1, 1 cannot be installed in parallel.